Killer Mouse on the Loose

On a South Atlantic island, home to the largest seabird colony in the world, a predator supermouse is creating havoc.

The Gough Island mouse is three times the size of a typical house mouse, and alone or in groups it preys on chicks sometimes 10 times its size, devouring them alive in their nests.

The gruesome phenomenon was first observed seven years ago, and has now so depleted the bird population of this World Heritage site that some of its species are facing extinction.

"We knew that when rats are introduced on an island with seabirds, the result is catastrophic," Royal Society for the Protection of Birds scientist Geoff Hilton told ABC News. "Mice however, had always arrived to pretty subtle effect."

Conservationists believe mice were brought onto the island in the 19th century by passing ships hunting seals.

The island is uninhabited, and without any competitors or predators -- neither cats or rats -- it appears the mice grew bolder and bigger, evolving from a strictly vegetarian diet to a carnivorous one, adopting ratlike behavior.

Ross Wanless, a researcher at the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, South Africa, who has witnessed the attacks, said they happen mostly in August, just after hatching season.

"We studied a population of about 60 birds last year," he told ABC News. "Some eggs didn't hatch, some chicks died at birth, but most were eaten. Only one chick survived."

Among the species most at risk of extinction is the Tristan albatross.

This rare breed is endemic to Gough Island, where more than 99 percent of its estimated 15,000 population lives. Even though albatross chicks weigh almost 300 times more than Gough Island mice, the rodents devour more than 1,000 of them every year.

Already threatened by long-line fishing, the Tristan albatross was placed on the International Union for Conservation of Nature's Red List of critically endangered species this weekend.

"The chicks don't react when mice nibble at them," Hilton said of the large bird.

"They evolved for thousands of years on an island deprived of mammals, so [they] didn't develop behavioral defenses," he said. "Effectively, they are being eaten from the inside without twitching, then weaken and die."

After birth, albatross chicks stay still for eight months, while being fed by their parents. It is the time when they are the most vulnerable.

"From a mouse point of view, it's a wonderful free meal of 50 percent fat that doesn't seem to move when you eat it," Hilton said. "It may take another thousand years for the birds to develop a defense system. Unfortunately, we don't have that time."

The Royal Society for the Protection of Birds was awarded $31,000 by the United Kingdom's Overseas Territories Environment Program to save the World Heritage island before it's too late.

Conservationists are working on a plan to air-drop a poisonous grain onto the island in order to eradicate the mice, a project that could cost more than $1 million. But the cost of the plan isn't the only concern: No one is sure it will work.

"If you get 99.9 percent of them, you wasted your money," Hilton said. "The mice will just come back."

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